1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, and, more particularly, to a nonreciprocal circuit device such as an isolator for use in the microwave band, and to a communication device.
2. Description of the Related Art
FIG. 13 is an electrical-circuit block diagram of an RF portion of a conventional portable phone 1. In FIG. 13, reference numeral 2 denotes an antenna element. Reference numeral 3 denotes a duplexer Reference numerals 4 and 6 each denote a power amplifier on the transmission side. Reference numeral 5 denotes an interstage band-pass filter on the transmission side. Reference numeral 7 denotes a mixer on the transmission side. Reference numeral 8 denotes a low-noise amplifier on the receiving side. Reference numeral 9 denotes an interstage band-pass filter on the receiving side. Reference numeral 10 denotes a mixer on the receiving side. Reference numeral 11 denotes an isolator. Reference numeral 12 denotes a voltage-controlled oscillator (VCO). Reference numeral 13 denotes a local band-pass filter.
Generally, the isolator 11 is disposed between the voltage-controlled oscillator 12 and the mixers 7 and 10 on the transmission and receiving sides in order to achieve isolation between the voltage-controlled oscillator 12 and the mixers 7 and 10 on the transmission and receiving sides, so that signals reflected from the mixers on the transmission and receiving sides are not returned to the voltage-controlled oscillator. Instead of the isolator 11, a buffer amplifier is sometimes used. However, since the isolator 11 of a nonreciprocal circuit device does not require power, the battery life is not affected by the isolator, so the isolator 11 has the advantage that the standby time and the telephone conversation time of the portable phone 1 can be increased.
As there has been a demand for smaller portable phones and to achieve cost reduction in recent years, ICs incorporating the mixer 7 on the transmission side and the mixer 10 on the receiving side (which are balanced input/output circuits) have become more and more common. However, the input/output ports of a conventional isolator 11 are both unbalanced-type ports. Therefore, in order to electrically connect to the isolator 11 the balanced-type input/output ports of an IC in which a mixer is incorporated, it is necessary to convert a parallel signal of the IC to a single ended signal by using a balun. For this reason, the number of components is increased, and the number of connection points is increased, presenting problems of radiation, resistive losses, and greater mounting area and failure rate, for example.
To address these problems, the present invention provides a nonreciprocal circuit device which can be connected to a balanced circuit without going through a balun, and a communication device.
The nonreciprocal circuit device according to the present invention may have two ports, wherein at least one of the two ports is a balanced-type port. For example, there are cases in which only the input port is a balanced-type port, or only the output port is a balanced-type port, or both the input port and the output port are balanced-type ports.
More specifically, the nonreciprocal circuit device may comprise a center electrode assembly formed of a ferrite and two center electrodes, a permanent magnet for applying a DC magnetic-field to the ferrite, and a metal case for housing the center electrode assembly and the permanent magnet. The balanced-type port is formed of a pair of terminals which are electrically connected respectively to the two ends of a line that is substantially a half-wave in length at an operating frequency, and one of the pair of terminals is connected to one of the center electrodes. Furthermore, the balanced-type port is formed of a pair of terminals which are electrically connected to both ends of one of the center electrodes via a matching capacitor.
The nonreciprocal circuit device having the above construction can be connected to a balanced circuit without going through a balun.
The communication device according to the present invention comprises a nonreciprocal circuit device having the above-described features, and therefore, high reliability can be obtained.
Further features and advantages of the present invention will become apparent from the following description of embodiments of the invention with reference to the attached drawings.